Outdoor Power Equipment and Small Engines - combustion question

Hey Cheese, can you please explain to me the difference between preignition and detonation. I never really understood the difference. Also, what are the usual signs of damage from the two? What causes the two? I know hot carbon deposites have something to do with one of them. Thanks again!!!!!!!

Good question Mower!

Detonation and preignition are the same thing. It is when the fuel ignites before it should, and before the ignition system fires the plug. It is usually caused by a glowing hot piece of carbon or metal burr inside the combustion chamber. It can also be caused by an overheated engine. If the wrong octane fuel is used in an engine that is hot enough and has enough compression, it will ignite the fuel the same way a diesel does.

I believe I may be experiencing some PRE-IGNITION in the 16HP Tecumseh I've been working on. Since sometimes there is a flame shooting OUT from the carb what I think is happening is this. There is a hot ignition source (perhaps one of those bumps you see in the picture) that stays hot through the exhaust cycle. Now, after the exhaust cycle completes the intake valve opens and and draws the fuel/air mixture into the cyclinder. The hot spot ignites the mixture inside the cylinder and it spread through the intake pipe out through the carb. The ignition is occuring BEFORE the compression even starts and while the intake valve is still open. Kinda a wierd thing to happen. Detonation happens when the fuel burns explosively rather than in an 'organized' flame front . This can occur due to overheating, a dirty combustion chamber, or due to the wrong gas. In order to generate HP you need high compression, but high compression requires high octane gas so you don't get pinging (knocking detonation). That's why race cars often don't use regular pump gas and sometime even resort to getting their fuel at airports where 130 octane aviation gas is (or used to) be available. It's the design characteristics of high octane fuel to resist combustion while enduring the heat of compression and burn in an 'organized' manner when ignited by the spark. A real good fuel that does this very well is alcohol. Even though alcohol had a lower heat of combustion you can go to a very high compression ratio in your engine and generate more HP without fear of detonation, which is a big contraint to gasoline engines. The designers of your engine have taken into account the combustion characteristics of the fuel when they design the engine. You can usually get away with using a higher octane than the engine is designed for (but you'll pay more than you need to) but if you use a lower octane you risk detonation and engine damage. Since detonation is the 'explosive' buring of fuel the sudden rise in combustion chamber pressures can overstress and damage engine components like pistons, rods, bearings, and crank shafts. You could have a problem with 'high test' fuel if the heat of combustion is higher because your engine my not be able to dump heat fast enough at higher throttle settings to keep from damaging components. Of course, like Cheese said, you can always get some PRE ignition (ignition of the fuel before the spark) due to hot spots or even (like in a diesel engine) ignition from the heat of compression.

To sum it up:

Preignition is combustion of the fuel BEFORE the spark and results in a mis-timed combustion chamber pressure rise and reduced power output.

Detonation is the 'explosive' combustion of fuel occuring AFTER the spark and results in a sudden rise in combustion chamber pressures (more than in normal combustion) that can cause structural damage to engine components.

At least that's the story I got while in school.